Implant for reconstruction of joints

ABSTRACT

A spacer member ( 1 ) is intended to be placed between the ends of the bones which are to be connected, one end of the spacer member being designed to form a joint surface against one of the bone ends ( 6,7 ). A joint-stabilizing connection ( 2,3 ) is arranged to connect the bones. The spacer member ( 1 ) is made of at least one tissue-compatible material.

This application is a division of co-pending application Ser. No.10/250,310, filed on Dec. 23, 2003. Application Ser. No. 10/250,310 isthe national phase of PCT International Application No. PCT/SE02/00038filed on Jan. 11, 2002 under 35 U.S.C. § 371, which claims priority ofSwedish Application No. 0100127-0 filed Jan. 15, 2001. The entirecontents of each of the above-identified applications are herebyincorporated by reference.

TECHNICAL FIELD

The present invention relates to an implant for reconstruction ofjoints, preferably of the hands and feet.

PRIOR ART

Primary wear, arthrosis, of the joints of the hands and feet, forexample the carpometacarpal joints of the thumb, is a common condition,especially in middle-aged women. Investigations show that about 12% ofall women and 8% of men in their fifties complain of pain at the base ofthe thumb. The incidence of wear of the big toe, hallux rigidus, isprobably even greater. These conditions cause pain at rest and alsoload-related pain and they reduce mobility. In the hand, this leads topain in various types of grips and thereby reduces the grippingstrength.

In the foot, wear causes pain when walking, with reduced mobility andfreedom of movement in the persons affected.

Today, arthrosis of the base of the thumb is initially treated withanti-inflammatory agents, local cortisone injections and various typesof supports. At a more developed stage of arthrosis of the base of thethumb, surgical methods are employed. In younger men, and in middle-agedmen in work, wear of the joints is preferably treated by an operationstiffening the joint.

Today, in middle-aged and older women, the surgical method employedinvolves some form of bridging graft with a tendon. However, tendoplastyrequires a long period of rehabilitation. The reduction in pain and theincrease in mobility are only achieved after 6 to 9 months. Moreover,the persons involved have to live with a permanent loss of strength inthe thumb grip.

Various types of prostheses have also been produced, for example fortreating arthrosis of the base of the thumb. They are made of titanium,steel, plastic, carbon or silicone. However, they fail after a shorttime on account of luxation of the joint. It has also been found thatwhen carbon is used, the carbon fibre structure breaks down mechanicallyover time and the carbon fibre fragments tend to migrate within thebody. U.S. Pat. No. 4,411,027 proposes surrounding the carbon fibrestructure with a shell of a bioabsorbable polymer for the purpose ofprotecting the structure from external mechanical action and keeping thefragments in place, at least in the early stage of healing. However, ithas been found that carbon fibre fragments from the prosthesis stillmigrate within the body.

SE-B-457,962 describes an implantable prosthesis for completely orpartially replacing a tendon, a ligament or a cruciate ligament. U.S.Pat. No. 6,007,580 further describes a prosthesis made of abiodegradable material, which prosthesis is intended to connect twobones.

However, no suitable implant for reconstruction of joints has hithertobeen proposed. Preconditions for such an implant to be able to functionwell are, first, that it must have properties, such as the requisitestrength and mobility, allowing it to replace the functions of thedamaged joint, and, second, that it is biocompatible, i.e. that the bodyis not poisoned or otherwise harmed by the implant. Joints areparticularly complicated since they involve joint capsules, ligaments,cartilage and synovial fluid in order to permit natural joint movements.

It has been stated above that joint damage at the base of the thumb isextremely common and represents a major problem and causes suffering andimpaired function in those concerned.

Another common problem is hallux rigidus, which is a form of arthrosisof the base of the big toe and which mainly affects men and entailsrestricted mobility of the metatarso-phalangeal joint of the big toe.This means that the foot cannot be deployed in the normal way whenwalking, and that the person affected suffers pain when walking and analtered gait pattern with loss of speed.

The abovementioned method with a bridging tendon graft means, in thecase of treating arthrosis of the base of the thumb, that a bone, namelythe trapezoid bone, is removed in its entirety, which means that thethumb is shortened by the order of 1 cm and that the short thumb muscleshave an altered range of functioning. Moreover, the proximal end of themetacarpal loses its stability. In order to reduce the instability andavoid conflict with the navicular bone, a bridging graft is formed froma tendon. However, the technique does not mean that the metacarpal iscompletely stable. This, together with the shortening of the thumb,leads to permanent loss of thumb strength both in the key grip and thethree-point grip.

As has been mentioned above, the prostheses which have hitherto beenproduced are not entirely satisfactory either. One reason is that knownprostheses do not stabilize the proximal metacarpal, which means thatthe prosthesis dislocates.

There has therefore long been a need for a solution to theabovementioned problems which would allow satisfactory reconstruction ofdamaged joints in humans and animals.

The present invention makes available an implant which is of the typeset out in the introduction and which completely eliminates theabovementioned problems.

The implant according to the invention is characterized in that leastone spacer member is intended to be placed between the ends of the boneswhich are to be connected, one end of the spacer member being designedto form a joint surface against one of said bone ends, ajoint-stabilizing connection is arranged to connect said bones, thespacer member is made of at least one tissue-compatible material, andthe joint-stabilizing connection is intended, upon use of the implant,to extend in the longitudinal direction across the joint and in over atleast one side of the two bones which are connected by the joint.

Since the implant comprises a spacer member, direct contact betweenadjacent bones is avoided, and thus also the occurrence of pain causedby bone rubbing against bone.

By means of the arrangement of said spacer member and ajoint-stabilizing connection it is possible to ensure that during theperiod of growth of new tissue the implant has the correct form and alsoappropriate strength and mobility.

According to one embodiment of the invention the material is porous,entirely or partly. This, in combination with a pore size permittingingrowth of new biological tissue means that the implant according tothe invention substantially recreates a functioning joint.

According to one preferred embodiment, the spacer member and the jointstabilizing connection are made of degradable material.

According to a further embodiment the joint-stabilizing connection ismade in one piece with the spacer member.

According to one embodiment the degradable material consists ofpolyurethane urea.

According to another embodiment the degradable material consists ofpoly-L-lactide.

According to a further embodiment the degradable material consists ofpolydiaxone (PDS).

According to another embodiment the degradable material consists ofpoly-β-hydroxybutyrate (PHB).

According to another embodiment the degradable material consists ofchitin or chitosan or polysaccharide.

According to a further embodiment the degradable material consists ofcollagen or protein.

According to a further embodiment the material consists of polyuretan.

According to a further embodiment the material consists of silicone.

According to a further embodiment the material consists of polyethyleneterephtalate (PET).

According to another embodiment the inventiontion is characterized inthat the implant in its entirety in cross-section mainly has the form ofa T, where the stem is said spacer member.

According to one embodiment the invention is characterized in that thespacer member includes a film-like element which is intended to serve assaid joint surface.

One embodiment of the invention is characterized in that the spacermember comprises a degradable and tissue-compatible material in the formof foam, fibre or thread, which material is cast, knitted or woven or insome other way formed to give the desired three-dimensional structure.

According to one embodiment, said connection consists of flexiblethread-like elements.

According to one embodiment, the invention is characterized in that thejoint-stabilizing connection consists of flexible thread-like elements,that said thread-like elements have at least one portion on both sidesof the centre line of the two bones, that opposite ends of each of saidportions are anchored in the respective bone, and that said portionsspan the joint and are designed to prevent mutual pivoting of the bonesin the lateral direction.

According to a further embodiment, the invention is characterized inthat said thread-like elements comprise two portions which each connectthe two bones, and that said portions are designed to intersect eachother across the joint, as a result of which movements in more than oneplane are permitted. When an implant according to this embodiment isarranged for reconstruction of a joint at the base of the thumb, thethumb can be moved in a more natural way during the healing process,which in turn means that growth of new tissue is stimulated, permittingan improved joint function for the thumb.

Further preferred embodiments are set out in the attached patent claims.

The invention will be described in more detail below with reference toillustrative embodiments which are shown in the attached drawing, where:

FIG. 1 shows a perspective diagrammatic view of a first illustrativeembodiment of an implant according to the invention.

FIG. 2 shows a cross section through an implant according to FIG. 1arranged in a joint between two bones.

FIG. 3 shows a section along the line III-III in FIG. 2.

FIG. 4 shows a cross section of an implant according to FIG. 1 arrangedin a joint between two bones, in a modified way compared to FIG. 2.

FIG. 5 shows a perspective view of the bones in a hand and wrist.

FIG. 6 shows a portion of the hand shown in FIG. 5, with an implantaccording to FIG. 1 arranged in a joint at the base of the thumb.

FIG. 7 shows a second illustrative embodiment of an implant according tothe invention arranged in a joint between two bones.

FIG. 8 shows a side view of the implant and bones according to FIG. 7.

FIG. 9 shows a view similar to FIG. 7 with an implant according to athird illustrative embodiment, slightly modified in relation to theembodiment according to FIG. 7.

FIG. 10 shows a side view of the implant and bones according to FIG. 9.

FIG. 11 shows a fourth illustrative embodiment of an implant accordingto the invention arranged in a joint between two bones.

FIG. 12 shows a side view of the illustrative embodiment according toFIG. 11.

FIGS. 13 and 14 show two sizes of spacer bodies which are included inthe implant according to the illustrative embodiments in FIGS. 7-12 andwhich are intended to form articular heads in a joint.

FIGS. 15 and 16 show two sizes of spacer bodies which are included inthe implant according to the illustrative embodiments in FIGS. 9 and 10and which are intended to form articular sockets in a joint.

FIGS. 17 and 18 show the structure of a spacer member in an implantaccording to a fifth embodiment.

FIGS. 19 and 20 show, in longitudinal section, the structure of a spacermember in an implant according to a sixth embodiment.

FIGS. 21 and 22 show the structure of a spacer member in an implantaccording to a seventh embodiment.

The implant in the illustrative embodiment according to FIG. 1 has aT-shaped cross section with a spacer member 1 and two connectionbranches 2, 3 which, during use of the implant, are intended to form ajoint-stabilizing connection. In the illustrative embodiment shown, theimplant is cast in one piece. Suitable materials for the implant arepolymers comprising urethane groups with hydrolyzable ester groups orpolymers comprising urea and urethane groups with ester links which arehydrolyzable. A suitable starting material for the implant is linearblock polymers comprising urea and urethane groups with hydrolyzableester groups. Material of this type is described in Swedish Patent505,703. The material according to this publication can be cast intoforms of the type shown in FIG. 1 or spun into fibres which are thenknitted or woven and shaped to give the desired finished product.Another suitable material is a network polymer which essentially lacksurea groups. Material of this type is described in Swedish Patent510,868. Implants made of said materials can also be formed by acombination of casting and knitting or weaving. For example, a spacermember can be formed by casting the polymer in question on areinforcement of the same material, which reinforcement can be designedas a hose or the like and intended to be engaged on a bone end. Bychoosing their structure and the molecular chains involved, and byadding various substances, the materials of said types described in saidSwedish Patents 505,073 and 510,868 can be controlled with respect totheir mechanical properties and also with respect to their degradationtime.

The implant according to the invention is intended to be used in thereconstruction of damaged joints in humans and animals. The geometricdesign and the mechanical properties are chosen in accordance with theintended purpose. The implant will temporarily replace the damaged jointand the time of degradation of the implant must exceed the time forformation of cartilage-like tissue in the spacer member andjoint-stabilizing connective tissue for forming a joint-stabilizingconnection. The implant is designed so that, during the rehabilitationperiod, it fulfils the intended joint functions with sufficient strengthand mobility. This, in combination with the fact that the spacer memberis porous and has a pore size permitting ingrowth of new biologicaltissue, means, as has been mentioned above, that the implant accordingto the invention stimulates recreation of a functioning joint, at thesame time as the implant is degraded. Material according to SE 505,703and SE 510,868 is broken down by hydrolysis and thereafter eliminatedfrom the body.

In FIG. 2, the implant according to FIG. 1 has been arranged in adamaged joint, for example a finger joint, between two bones 4, 5. Thespacer member 1 is arranged between the bone ends 6, 7 of the respectivebones, by which means direct contact between adjacent bone ends isprevented and pain caused by bone rubbing against bone is avoided.Before the implant has been fitted, recesses 8 have been formed in thebones 4, 5 and are intended to accommodate the joint-stabilizingconnection formed by the branches 2, 3. The bone end 7 serves as anarticular socket and the bone 5, in the position shown in FIG. 2, ispivotable in the direction of the arrow A about the temporary jointsurface which is formed by the spacer member 1. The implant is flexiblebut substantially non-stretchable during normal use, i.e. under theforces which occur when a finger joint is bent in a natural manner. Thebone 5 can thus be bent from the position shown in FIG. 2 by the orderof magnitude of 90° and back, but not clockwise from the position shownin FIG. 2. The opposite surface of the spacer member is in contact witha bone end 6 serving as an articular head on the bone 4, and the bone 4can pivot correspondingly in relation to the bone 5 in the direction ofthe arrow B. The spacer member, like the rest of the implant, is porouswith a pore size which permits ingrowth of new cartilage tissue forcontinuous re-formation of a permanent joint. The pore size also permitsingrowth of joint-stabilizing connective tissue in the branches 2, 3 ofthe implant, for continuous replacement of the temporaryjoint-stabilizing connection. Re-formation of new cartilage tissue forthe spacer body takes something in the region of 1½ to 2 years and thetotal degradation time for the temporary spacer body must thereforeexceed this period of time. The connective tissue which replaces thetemporary connection is re-formed in a shorter time than cartilagetissue, and it takes something in the region of six months to 1½ yearsfor connective tissue to completely re-form.

In the illustrative embodiment shown in FIGS. 2 and 3, the implant hasbeen secured in the bones 4, 5 by means of suture threads 9 which, as isshown in FIG. 3, connect the branches 2, 3 of the implant to therespective bones.

In the illustrative embodiment shown in FIG. 4, details corresponding tosimilar ones in the illustrative embodiment according to FIGS. 2 and 3have been provided with the same reference numbers. Compared to FIGS. 2and 3, FIG. 4 shows an alternative means of securing the implantaccording to FIG. 1. Instead of the recess 8, grooves 10 for thebranches 2, 3 have been formed in the bones 4 and 5, as can be seen fromFIG. 4. These branches, which form a joint-stabilizing connection, havebeen secured in the bones 4 and 5, respectively, by means of continuousscrews 9′. In the method according to FIG. 4, an uninterrupted hardouter bone surface is obtained on both the bones 4 and 5.

As was stated in the introduction, primary wear, for example in thecarpometacarpal joints of the thumb, is a common type of condition whichcauses degradation and attrition of joint cartilage. FIG. 5 shows thebones of the hand from above, i.e. the back of the hand. In the drawing,the 1st metacarpal has been indicated by 12 and the trapezoid bone by13. Said joint condition, namely arthrosis of the base of the thumb,occurs in the joint between the 1st metacarpal 12 and the trapezoid bone13.

FIG. 6 shows how an implant according to FIG. 1 can be arranged betweensaid bones 12 and 13. In the example shown, a worn portion of thetrapezoid bone 13 has been removed and the spacer member 1 has beenarranged between this sectioned bone and the 1st metacarpal 12. Thespacer member 1 bears tightly on the surface of the sectioned bone. Thebranches 2, 3 have been connected by means of suture threads 9 to therespective bones 12 and 13 in order to form a joint-stabilizingconnection at the base of the thumb.

FIGS. 7 and 8 show a second illustrative embodiment of an implantaccording to the invention arranged in a joint between two bones 4 and5, for example in a finger or a toe. A spacer member 14 is arranged inthe joint between the two bones. This spacer member 14 is preferablymade of the same material as the implant according to the firstembodiment. The spacer member 14 of the type included in the implantaccording to FIGS. 7 and 8 is shown separately in FIGS. 13 and 14. Aspacer member of this type can be produced in different sizes andthicknesses. FIG. 13 shows a very thin spacer member 14, and FIG. 14shows a very thick spacer member 14. For joints in the hands and feet,the necessary thickness of the spacer member varies in the range of 0.5to 7 mm. For joints in animals, the thickness of spacer bodies of thetype shown in FIGS. 13 and 14 can preferably vary within wider limits.The spacer bodies 14 are provided with securing portions 15 which areexpediently made of the same material as the rest of the spacer body.The securing portions 15 according to FIG. 13 are made for example aswoven or knitted ligaments. Alternatively, the securing portions canconsist of suture threads or some other degradable, preferablybio-erodable material. The securing portions 15 are used for anchoringthe spacer member 14 on one of the bones. FIG. 7 shows diagrammaticallyhow the securing portions 15 have been connected to the bone 4 by meansof suture threads 9. The implant according to FIGS. 7 and 8 includes aflexible thread-like element 16 which is intended to serve as atemporary joint-stabilizing connection. As can be seen from FIGS. 7 and8, the thread-like element has a portion 17, 18 on both sides of thecentre line of the two bones, which portions extend in the longitudinaldirection of the bone and over the joint. In this illustrativeembodiment, the thread-like element is made in one piece and extendswith portions 19 and 20 through holes in the bones 4, 5, respectively.

As in the embodiment according to FIGS. 2 and 3, the bones 5 and 4 canbe bent from the position shown in FIG. 8 in the direction of the arrowsA and B, respectively. The thread-like element is made of a degradable,preferably bio-erodable material, preferably of a linear block polymerof the type which has been described above, and has further beendesigned with mechanical properties similar to a natural ligament withinits natural range of movement. This means that the thread-like elementhas, at least within a normal ligament's range of functioning, similarmechanical properties but that beyond this range of functioning it maybe overdimensioned compared to a normal ligament. The thread-likeelement 16 is flexible but substantially non-stretchable, which meansthat the bones 4, 5 in the position shown in FIG. 8 cannot be bent aboutthe joint in the direction counter to the arrows A and B, respectively.

A third illustrative embodiment according to FIGS. 9 and 10 correspondsto a large extent to the embodiment according to FIGS. 7 and 8.Corresponding details in said embodiments have been provided with thesame reference numbers. What distinguishes the implant according toFIGS. 9 and 10 from the embodiment described in connection with FIGS. 7and 8 is that a further spacer member 21 has been arranged against thebone end 7 of the bone 5. Spacer members of this type and designed as anarticular socket are shown separately in FIGS. 15 and 16. In the sameway as has been described in connection with FIG. 13 and 14, thethickness of the spacer member can be varied within the same limits.Moreover, the spacer members designed as articular sockets are providedwith securing members 15 which are intended to anchor the spacer body 21on the bone 5 by means of suture threads 9, as shown in FIGS. 7 and 8.

FIGS. 11 and 12 show a fourth illustrative embodiment of an implantaccording to the invention. This fourth embodiment differs from theillustrative embodiment shown in FIGS. 7 and 8 only with respect to thethread-like element. This has been arranged with thread portions 22, 23intersecting across the joint between two bones 4, 5, as can be seenfrom FIGS. 11 and 12. The thread-like element is made in one piece 16and has portions 24, 25 which extend through holes in the bones 4 and 5.Since the thread portions 22 and 23 run more freely on one side of thebone compared to the embodiments described in connection with FIGS. 7 to10, where respective thread portions 17 and 18 run on both sides of thelongitudinal centre line of the bones and prevent mutual pivoting of thebones in the lateral direction, the bones 4 and 5 can, in addition tothe possibility of pivoting in the direction of the arrows A and B inFIG. 12, also be pivoted in another plane, such as is indicated by thearrows C in FIG. 11.

FIG. 17 shows a knitted or otherwise formed network structure 26 made ofa degradable material, preferably from said linear block polymersaccording to SE 505,703. Said structure has the shape of a hood and isintended to serve as reinforcement in an implant. The latter is formedby means of a degradable material, preferably in the form ofpolyurethane with hydrolyzable ester groups, being cast from outsideonto the top of the hood. Said material is described in SE 510,868.

FIG. 18 shows the finished spacer member after casting, intended toserve as an articular head in a joint, for example a finger joint. Thecast-on material has been indicated by 27 in FIG. 18. The networkstructure 26 forms a hose which strengthens the spacer body from theinside, and the network portion 28 protruding from the cast body 27forms a hose portion which is intended to be engaged on a bone which isto be connected to an adjacent bone via the joint. A further purpose ofthe network structure, besides that of serving as a strengthening means,is to create a high degree of friction against the bone and facilitategrowth therein. The network structure 26 is also stable for sewing inupon fixation to the bone.

FIG. 19 shows, in a similar way to FIG. 17, a knitted or otherwiseformed network structure 29 which is intended to form an outerreinforcement for a spacer member, which is intended to serve as anarticular socket, i.e. as a complement to a spacer member of the typewhich has been described in connection with FIGS. 17 and 18.

The network structure 29 is provided with one or more securing portions30 which are intended to be secured on the outside of a bone by means ofsuture thread or the like. The securing portion 30 can go all the wayround or be made up of one or more securing portions.

An illustrative embodiment with two securing portions 30 is shown in thedrawing.

A degradable material, preferably in the form of polyurethanes withhydrolyzable ester groups, is cast onto the inside of the networkstructure, as can be seen from FIG. 20, for forming an articular socket.The cast-on material has been indicated by 31 in FIG. 20. The purpose ofthe network structure according to FIG. 20 is the same as has beendescribed in connection with FIG. 18, i.e. to serve as reinforcement inthe spacer member, create friction against the bone and facilitategrowth in the bone.

FIGS. 17 to 21 show relatively thin spacer members corresponding to thespacer members shown in FIGS. 13 and 15.

FIGS. 21 a and 21 b show a knitted or otherwise formed network structure32 of the type which has been described in connection with FIG. 17above.

The network structure 32 differs from that shown in FIG. 17 in that thetop 33 of the structure is condensed and in that the reinforcement inthe top 33 has a depth which will substantially correspond to thethickness of the degradable material 34, see FIG. 22, which is cast fromthe outside and in across the depth of the network structure in the top33. The cast material 34 like the network structure is expediently madeof polyurethanes with hydrolyzable ester groups.

The actual spacer member in the embodiment according to FIG. 22 has athickness which substantially exceeds the thickness of the spacer memberaccording to FIG. 18. In principle, the difference arises in the sameway as the difference in size which has been described in connectionwith FIGS. 13 and 14 above. The network structure 32 fulfils the samepurposes as have been described above in connection with FIGS. 17 to 20.

However, the network structure 32 has a substantial depth at the top 34and in this area it will serve also as internal reinforcement in thecast-on material. It is important that the spacer member in a joint doesnot collapse under the pressures which arise during natural use of thejoint, and instead the temporary spacer member must hold adjacent boneends at a distance from each other. It is of course also important thatthe temporary spacer members are resistant to wear.

The different embodiments of the spacer members shown in FIGS. 17 to 22are combined with a joint-stabilizing connection, for example in theform of flexible thread-like elements which have been described inconnection with FIGS. 7 to 12.

The invention is not limited to the illustrative embodiments describedabove, and instead it can be modified in a number of ways within thescope of the attached patent claims.

The spacer member, which can consist of a cast or spun and thereafterknitted or woven structure or an otherwise formed structure, can beprovided, as has been described above, with strengthening threads of thesame material as the rest of the structure or of another degradable andtissue-compatible material.

Said strengthening threads can be arranged to form a reinforcementskeleton which is intended to give the structure stability withoutlocking it. The strengthening threads can for example be arranged in adefined direction in order to afford increased strength in thisdirection.

The reinforcement skeleton can consist of a shell which surrounds therest of the structure. In addition to said shell, the reinforcementstructure can also comprise strengthening threads in one or more defineddirections.

In the above described embodiment have been mentioned suitable examplesof materials for a degradable implant. Further examples of degradablematerials for the implant in accordance with the invention are definedin the following patent claims.

The invention is not restricted to implants of degradable materials.Examples of non-degradable materials are silicone and polyethyleneterephtalate.

The implant according to the invention can have different forms thanhave been described above. All corners of the in the figuresschematically shown T-shaped implant are preferably rounded.

The implant can be shaped as a plate with integrated fastening membersor joint-stabilizing connections. The implant can have a sponge-likeshape with integrated stabilizing connections.

Within the scope of protection is an implant arranged such that theshape of the implant is adapted to the anatomic shape when exposed toload.

The joint-stabilizing connection can be designed in a different way thanhas been described above.

The material of the spacer member can consists of polytetrafluoroethylene (PTFE). Also the joint-stabilizing connection can be made inthe same material. This material is generally known under the trade nameTEFLON®.

1. Implant for reconstruction of joints, which is T-shaped in crosssection and is made entirely of degradable polyurethane urea which is ofone piece construction and at least partly porous.